
112
7593L–AVR–09/12
AT90USB64/128
strobe. Therefore it is the value present in the COM0B1:0 bits that determines the effect of the
forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR0B as TOP.
The FOC0B bit is always read as zero.
Bits 5:4 – Res: Reserved bits
These bits are reserved bits and will always read as zero.
Bit 3 – WGM02: Waveform Generation Mode
Bits 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the
counter even if the pin is configured as an output. This feature allows software control of the
counting.
14.8.3
TCNT0 – Timer/Counter Register
The Timer/Counter Register gives direct access, both for read and write operations, to the
Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare
Match on the following timer clock. Modifying the counter (TCNT0) while the counter is running,
introduces a risk of missing a Compare Match between TCNT0 and the OCR0x Registers.
14.8.4
OCR0A – Output Compare Register A
Table 14-8.
Clock Select bit description.
CS02
CS01
CS00
Description
0
No clock source (Timer/Counter stopped)
001
clkI/O/(No prescaling)
010
clkI/O/8 (From prescaler)
011
clkI/O/64 (From prescaler)
100
clkI/O/256 (From prescaler)
101
clkI/O/1024 (From prescaler)
1
0
External clock source on T0 pin. Clock on falling edge.
1
External clock source on T0 pin. Clock on rising edge.
Bit
7
65
43
21
0
TCNT0[7:0]
TCNT0
Read/write
R/W
Initial value
00
Bit
7
65
43
21
0
OCR0A[7:0]
OCR0A
Read/write
R/W
Initial value
00